This invention relates to an apparatus for converting rotational motion to linear motion which utilizes a screw shaft and a nut, both having helical threads and between which are interposed threaded planetary rollers. More particularly, the planetary rollers, and shaft and/or nut are provided with a threaded locking system to provide stabilized rotation.
Prior to the present invention, roller screw drives have been available which comprise a screw shaft axially positioned within a nut and wherein a plurality of planetary rollers are positioned between the screw shaft and the nut. The rollers mesh with a helical thread on the nut and a helical thread on the screw shaft. The helix on the nut and the screw shaft extend in the same helix direction. In addition, the shaft and the nut have the same lead (distance advanced per revolution) and the same number of starts (beginning of a helical thread). When the nut is rotated, the shaft is caused to move axially with respect to the nut and the rollers. The rollers may or may not move axially with respect to the nut. As used herein, the term "start" is defined as the beginning of a helical thread in the right-hand screw thread direction or clockwise moving away from the observer. The term "negative start" is defined herein as the beginning of a helical thread in the left-hand screw thread direction or counterclockwise moving away from the observer. The terms "helix" and "helical thread" is defined herein to include the degenerate case where a helix becomes a circle and a helical thread with zero starts is a series of equidistant grooves perpendicular to the axis of symmetry of the part. Thus, as used herein, a nut and screw will have equal starts when each has the same number of starts and the starts extend in the same direction.
Presently available roller screw drives utilize a nut and a screw having the same number of starts. In order to maintain planetary rollers having starts, within the nut during rotation, the ratio of the nut diameter to the roller diameter is made equal to the ratio of starts. In this design, additional provisions are required to maintain a non-slip condition between the rollers and the nut. This consists of spur gears cut into the ends of each roller and mating ring gears installed at each end of the nut. High gear ratios (small lead) may be obtained when utilizing a very fine thread design: i.e., a large number of threads per unit length which is impractical to produce due to the very tight tolerances. Furthermore, when utilizing a nut and screw shaft, the threads of which extend in the same helical direction, with planetary rollers having starts, a large helix angle mismatch of the shaft threads with the planetary rollers occurs which results in poor load carrying capability.
When utilizing a nut and screw shaft, the threads of which extend in the same direction with planetary rollers having no starts, it is mandatory that the rollers be recirculated; i.e., that the rollers be removed from engagement with the nut and screw shaft and be moved axially to return the planetary roller to a former position so that the roller will be retained between the nut and screw shaft during extended rotation times. This mode of operation causes radially asymmetrical static forces and radial and axial dynamic imbalances that prevents high speed rotation. Skidding of the rollers relative to the shaft and nut causes damage during quick acceleration or deceleration.
Many of the presently available roller screw drives also have the problem that the planetary rollers do not remain intrinsically spaced apart from each other during rotation. Therefore, roller cages are employed to maintain uniform roller spacing. This is necessary to provide load sharing between rollers and to prevent jamming or excessive roller to roller friction.
Accordingly it would be desirable to provide a roller screw drive which permits obtaining a high gear ratio (small lead) without the necessity of utilizing a fine thread design. Furthermore, it would be desirable to utilize a roller screw design which does not require extreme thread helix angle mismatch between the planetary rollers and the nut and screw. Also, it would be desirable to have a roller screw design that utilizes planetary rollers which does not require roller recirculation. Also it would be desirable to provide a means for maintaining the desired spaced-apart relationship of the planetary rollers of a roller screw during rotation in order to provide balanced load sharing and to prevent jamming and roller to roller friction. Also it would be desirable to simplify the design and reduce the number of parts and their cost and complexity. It would be desirable to accomplish all this without having to restrict the range of axial travel between different parts of the roller drives.